Add a comment to gen_initramfs_list.sh output that contains the
  numeric mtime of the last modified file in the source directory,
  or the mtime of the source list.  This should cause the initramfs
  image to be rebuild if a file in the source directory changed, or
  the source list (if that was used rather than a directory as source).
Signed-off-by: Martin Schlemmer <azarah@nosferatu.za.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>